linux-kernel - Re: [PATCH v2] net: phylink: add missing supported link modes for the fixed-link

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Message-ID: <aRs8gMOyC9ZbqMfe@shell.armlinux.org.uk>
Date: Mon, 17 Nov 2025 15:17:20 +0000
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Wei Fang <wei.fang@....com>
Cc: andrew@...n.ch, hkallweit1@...il.com, davem@...emloft.net,
	edumazet@...gle.com, kuba@...nel.org, pabeni@...hat.com,
	eric@...int.com, maxime.chevallier@...tlin.com, imx@...ts.linux.dev,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] net: phylink: add missing supported link modes for
 the fixed-link

On Sun, Nov 16, 2025 at 10:38:23AM +0800, Wei Fang wrote:
> Pause, Asym_Pause and Autoneg bits are not set when pl->supported is
> initialized, so these link modes will not work for the fixed-link. This
> leads to a TCP performance degradation issue observed on the i.MX943
> platform.
> 
> The switch CPU port of i.MX943 is connected to an ENETC MAC, this link
> is a fixed link and the link speed is 2.5Gbps. And one of the switch
> user ports is the RGMII interface, and its link speed is 1Gbps. If the
> flow-control of the fixed link is not enabled, we can easily observe
> the iperf performance of TCP packets is very low. Because the inbound
> rate on the CPU port is greater than the outbound rate on the user port,
> the switch is prone to congestion, leading to the loss of some TCP
> packets and requiring multiple retransmissions.
> 
> Solving this problem should be as simple as setting the Asym_Pause and
> Pause bits. The reason why the Autoneg bit needs to be set is because
> it was already set before the blame commit. Moreover, Russell provides
> a very good explanation of why it needs to be set in the thread [1].
> 
> [1] https://lore.kernel.org/all/aRjqLN8eQDIQfBjS@shell.armlinux.org.uk/
> 
> Fixes: de7d3f87be3c ("net: phylink: Use phy_caps_lookup for fixed-link configuration")
> Signed-off-by: Wei Fang <wei.fang@....com>
> Reviewed-by: Maxime Chevallier <maxime.chevallier@...tlin.com>

Even though discussion is still going on, we do need to fix this
regression. So:

Reviewed-by: Russell King (Oracle) <rmk+kernel@...linux.org.uk>

Thanks!

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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